1. Field of the Invention
The present invention relates to an optical device, in particular to an image detector.
2. Description of the Related Art
As is known, in today's electronic industry, the trend towards integrating different devices in a single multipurpose system is increasingly marked. For example, the mobile-telephone field is undergoing an extremely rapid evolution, and one of the most important developments envisages the integration of miniaturized videocameras in cellphones.
To this aim, it is necessary to design sensor units, which, on the one hand, have reduced overall dimensions and are simple and inexpensive to produce and, on the other hand, have optical characteristics such as do not impair the quality of the images that are detected.
For greater clarity, reference is made to FIG. 1, wherein an image-detecting device 1 is illustrated, which comprises a sensor 2, a supporting element 3, and an objective 4.
The sensor 2, for example a matrix CMOS sensor, is formed in a semiconductor material chip and is carried in a housing 5 of a ceramic body 6. A glass layer 8, which closes the housing 5 at the top and is arranged at a distance from the sensor 2, seals the sensor 2 without altering the optical properties of light beams directed towards the sensor 2.
The supporting element 3, of plastic material, is rigidly coupled with the sensor 2 and the ceramic body 6; for example, it is bonded on the glass layer 8, and has inside it a threaded seat 9.
The objective 4 comprises a barrel 10 and an optical unit 11, interference-fitted together.
In particular, the optical unit 11, formed of one or more lenses, not illustrated herein in detail, has a cylindrical shape and an optical axis A, which is also a longitudinal axis of symmetry. The optical unit 11 is forced inside a cavity 13 of the barrel 10 and is coaxial with the barrel 10. In addition, the barrel 10 has a bottom wall 14 that delimits the cavity 13 and has a through opening 15 enabling passage of light towards the sensor 2, and an externally threaded side wall 17, which can be connected to the threaded seat 9 of the supporting element 3 so as to form a threaded connection, which has a dual function. First, this connection enables precise adjustment of the axial position between the optical unit 11 and the sensor 2, which are rigidly coupled, respectively, with the barrel 10 and with the supporting element 3. In this way, it is possible to focus the objective 4 on the sensor 2. In addition, the connection between the supporting element 3 and the barrel 10 has the function of keeping the optical unit 11 properly aligned. In particular, in ideal conditions, the optical axis A must be exactly orthogonal to the plane defined by the upper surface 2a of the sensor 2.
The accuracy of the image-detecting device 1 is markedly influenced not only by correct focusing, but also by the precision with which the optical axis A of the optical unit 11 is centered and aligned with respect to the sensor 2. Centering, alignment and focusing are carried out in the factory. Operatively, the optical axis A is centered by translating the supporting element 3 parallel to the upper plane surface 2a of the sensor 2, after the supporting element 3 and the objective 4 have been pre-assembled and before the supporting element 3 is bonded on the glass layer 8. After bonding, the threaded connection between the supporting element 3 and the barrel 10 is exploited to carry out focusing. As mentioned above, in this step the barrel 10 is rotated so as to adjust appropriately the axial position of the optical unit 11 with respect to the sensor 2.
In practice, however, the threaded connection between the supporting element 3 and the barrel 10 inevitably has some play causing the alignment of the optical axis A of the optical unit 11 to be imprecise with respect to the axis of symmetry (not shown) of the support 3. As illustrated schematically and in an exaggerated way in FIGS. 2a, 2b, in fact, the misalignment of the optical axis A with respect to the axis of the support 3 and hence the lack of orthogonality of the optical axis A with respect to the surface 2a may cause, during the rotation of the objective 4, a precession movement of the optical axis A. In addition, also centering is impaired. Very frequently, the deviation of the optical axis A from the correct position can be considerable, and hence the image detected by the sensor 2 is distorted and of unacceptable quality.
The above drawback is particularly serious, in so far as it can be detected only during focusing and hence after bonding the supporting element 3. Consequently, a alignment defect in the optical axis A renders the entire device 1 unusable, and not only the objective 4, thus leading to significant repercussions on the yield of the production process.